1. Technical Field
This invention relates generally to laser scanners for various applications, including an obstacle avoidance system for a combat helicopter. More particularly, it relates to a holographic optical element suitable for large-beam scanning and suppression of stray light noise.
2. Background Information
Recall that some scanners scan a large angular area with a beam of small angular divergence by mechanically moving portions of the beam-forming optics, usually flat mirrors. The scanner usually uses two sets of mechanically oscillated mirrors for a rectangular scan pattern. One mirror oscillates rapidly to deflect the beam along a line in one direction (e.g., vertically), while the other oscillates more slowly to move the line in a perpendicular direction (horizontally) to produce coverage similar to that in video display technology. Another method involves rotating multifaceted mirrors, each deflecting the beam into a given direction. Still another method deflects the beam with rotating or oscillating prisms.
Each of those methods has certain disadvantages. The oscillating-mirror method usually requires two mirrors, and the oscillations introduce vibrations, time delays, and non-linear rates, as well as limiting the maximum repetition rate. The faceted-mirror method requires mirrors large enough to encompass the entire beam, and that can be prohibitively expensive for large beams. Deflection by prisms is usually practical for only small beams, and small angles, since the weight of the prism becomes large for other conditions. In addition, scanning at high repetition or rotational rates may impart severe stresses in bulky conventional optical elements.
Holographic optical elements (HOEs) can overcome some of those disadvantages. A HOE includes one or more holograms that alter the direction of light rays by diffraction rather than refraction or reflection. It functions essentially independent of substrate geometry. So, the HOE can be made thinner and lighter. It can be replicated inexpensively through a simple photographic process to significantly reduce production costs compared to the grinding and polishing of conventional optical elements. In addition, multiple holograms can be superimposed into a single volume-hologram film to achieve multifunction HOE capability.
HOEs have previously been used for scanning in such applications as point-of-sale terminals by rotating multiple small HOEs into position to deflect a small beam. However, the structure required can be impractical for a larger beam that is used to achieve eye-safety and to obtain high resolution images with low beam divergence. In addition, point-of-sale terminals have no need nor provision for a high signal-to-noise ratio. So the HOEs can be inefficient without concern for the beam energy passing straight through. But this undeviated, bleed-through transmittance of the beam (the zero-order beam) and other stray light can cause noise, undesired ghost images, and other false signals in laser rangefinding. Thus, laser rangefinding needs a better way to take advantage of HOEs.